The present invention relates to a digital-to-analog converter (hereinafter referred to as xe2x80x9cD/A converterxe2x80x9d) and a display unit with such a D/A converter, and more particularly to a reference-voltage-selection-type D/A converter and a display unit such as an LCD (liquid crystal display) unit or an EL (electroluminescent) display unit with a drive circuit including such a D/A converter.
In recent years, liquid crystal display units tend to have a digital interface drive circuit integrally formed on the same panel as a pixel assembly. In the liquid crystal display units with the integral drive circuit, the pixel assembly comprises a matrix of pixels using polysilicon TFTs (thin-film transistors) as switching elements, and a horizontal drive system and a vertical drive system are disposed around the pixel assembly. The horizontal and vertical drive systems comprise TFTs and integrally formed with the pixel assembly on the same substrate (hereinafter referred to as xe2x80x9cLCD panelxe2x80x9d).
One serious problem with the fabrication of the above liquid crystal display units with the integral drive circuit is that the digital interface drive circuit integrally formed on the LCD panel takes up a large area around the pixel assembly (such an area will hereinafter be referred to as xe2x80x9cframexe2x80x9d). Particularly, in liquid crystal display units with an integral drive circuit which has a reference-voltage-selection-type D/A converter, the D/A converter takes up a large area which poses a significant problem on efforts to make the frame of LCD panels smaller.
Specifically, the reference-voltage-selection-type D/A converter comprises transistor switches between reference voltage lines which supply a plurality of reference voltages and a column line of the pixel assembly and the switch portions take up a large area in the D/A converter. Since there are required as many reference voltage lines as the number of gradations, these reference voltage lines take up a large area, i.e., the reference voltage lines extending up to the D/A converter in the LCD panel take up a large area. This large area presents an obstacle to attempts to reduce the size of the frame of LCD panels, and is responsible for making it difficult to increase the number of gradations.
It is therefore an object of the present invention to provide a D/A converter which allows the frame of a display panel to be reduced in size, and a display unit which includes such a D/A converter.
To achieve the above object, there is provided in accordance with the present invention a digital-to-analog converter comprising reference voltage generating means for generating 2n reference voltages corresponding to n-bit (n is an integer of 2 or greater) digital data, 2n reference voltage lines for transmitting the 2n reference voltages, respectively, generated by the reference voltage generating means, and 2n gradation selecting units connected between the 2n reference voltage lines and an output line, for operation depending on a logic state of each of bits of the digital data. The 2n gradation selecting units comprises transistor switches which have different transistor sizes depending on a selected gradation, or the 2n reference voltage lines have different line widths depending on a selected gradation.
According to the present invention, there is also provided a digital-to-analog converter comprising reference voltage generating means for generating a plurality of reference voltages, resistive voltage-dividing means for generating 2n reference voltages corresponding to n-bit (n is an integer of 2 or greater) digital data based on the reference voltages generated by the reference voltage generating means by way of resistive voltage division, and a gradation selecting circuit for selecting and outputting one of the 2n reference voltages supplied from the resistive voltage-dividing means, depending on a logic state of each of bits of the digital data. The resistive voltage-dividing means has voltage-dividing resistors having different resistances depending on a selected gradation, or the resistive voltage-dividing means directly outputs the reference voltage for a particular gradation supplied from the reference voltage generating means as one the 2n reference voltages.
The digital-to-analog converter is used in a display unit such as a liquid crystal display unit or an electroluminescent display unit which has a pixel assembly comprising a matrix of pixels. The digital-to-analog converter is incorporated in a digital interface drive circuit for selecting a desired reference voltage corresponding to digital data from as many reference voltages as the number of gradations and outputting the selected reference voltage as an analog signal. Particularly, the digital-to-analog converter is used as a digital-to-analog converter in a digital interface drive circuit disposed around the pixel assembly on the same substrate as the pixel assembly.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.